Electrolytic process for the purification of acetic acid solutions



y 1932- I c. s. WEBBER ET AL 57,224

ELECTROLYTIC PROCESS FOR THE PURIFICATION OF ACETICACID SOLUTION Filed Jan. 20, 1930 Patented May 10,

UNITED STATES Pm-m orrics was a. Warm or srmcrmr.n, mssacnusmrs, mm mm J. s'rnm um i man! as is. our, or nocnasraanawronx, assmnons 'ro mama xonax comm, or 30mm, m you, a coarom'rxon or new YORK I WIIHIC PROCESS FOR THE PUBIIIIGATION OI ACE'IIC LCD) SOLUTIONS v Application fled January :0, mo. Serial in. arm.

. 1 therefore, included in the mixture. In order to effect the acetylation of cellulose in an economical manner itis necessary that the acetic acid added to and that formed during the esterifi cation,.,of the cellulose be removed from the cellulose acetate, recovered 10I1S 1n solution or the speeds at which these and purified for further use.

In other manufacturing processes in which acetic acid is emplpfyedand in which ingredients necessary to e ect the required reactions 5 are added thereto, it is likewise essential that some means for removing these substances, and thereby rendering the acetic acid of sufiicient purity for its use in subsequent reactions, be found. 0 a After the separation of the acetic acid from the product formed, such as cellulose acetate from an acetylation bath, it would seem natural to effect this purification of the acetic acid by a simple distillation rocess. Such a method of purification woul not rove satisfactory nor could the purification conducted in a continuous, manner successfully, due tothe presence therein of the contaminating mineral acid, metal salts and other dissolved substances. It has been found desirable, therefore, to remove from the acetic acid prior to its distillation these contaminating constituents but up to timeof the invention hereinafter disclosed no entirely successful method for their removal had been found. Fig. 1 diagrammatically illustrates a discontinuous apparatus or electrolytic cell for ca out this purification process.

' Fig. 2 is likewise a diagrammatic drawing of a in which the impurities may reprovide a process for the electrolytic purification of acetic acid. Another object of this mventlon 1s the electrolytic purification of acetic acid solutions whereby the contaminating mineral acid salts and metallic, and I other ionizable impurities contained in solution may be removed therefrom. Other ob.j[ects hereinafter ap ear.

t 1s well known that t e anions of the organic acids, such as acetic acid, have a relatively low mobility in solution,that is, the ion (insolution, of course) when subjected to an electrolytic current moves in that solution. at a relatively slow rate when compared withthe anions of articularly the mineral acids which have a h mobillty.

The velocity of the chlori e and acetate ions travel when subjected to an electropotential of forinstance one volt the electrodes being one centimeter apart and the temperature of the aqueous solution being 18 C., is, for the chloride ion 2.12 centimeters per hour and for the acetate ion 1.04

centimeters per hour; see Introduction to .Physical' Chemistry, James Walker, Macmillan & Co., 1907, page 231. While it is not necessary that the degree of difierence between the mobility of the two ions be as great as that between the chloride and acetate and ra id will be their separation. A difierence 0 approximately 20% between the rates ofmobility of the 10118 being separated is sufiicient for the successful operation of our process nevertheless if several cells are used in series ions havin an even lower difierence may be succes ully separated. As the acetic acid" solutions recovered from an acetylation process generally contain hydroit is possible to rapidly and thoroughly re move from the acetic acid solutions the more required for the most distant rapidly moving anion to reach the anode portion of the cell, it is readily seen that any particular'cell may be used in which the distance the anion must travel is reduced to a minimum.

From the above description of our process -it is evident that many forms of apparatus may be employed for its successful operation, the two forms shown in the accompanying drawings being illustrative of typical species having exceptional advantages. Fig. 1 shows cell l which is constructed of any type of noncorrodible metal such as stainless steel or chromium plated steel or other metal or, for

a that matter, of enamelled or porcelain ware in which is placed the cathode 2 which covers the inside wall of the vessel and which may be constructed of brass or copper or other Tally serviceable cathode material upon w ich the'metallic salts may beplated. In

I the center of the cell is placed-the anode com partment 3 which is surrounded by a porous I cell 4 suspended inthe center of which is the anode 5. Wires 6 and 7 conduct the negative and ositive current from a power source, not s own, to the cathode and anode respectively. 1 The anode may be constructed of a material which is not attacked by the anions which are drawn to it and we have found for this purpose that anodes of carbonor platinum are uite suitable.

Fig. 2 iagrammatically illustrates a continuouwoce'ss for effecting this purification. ile we here show but one cell a number of these cells in series may be utilized I to thoroughly and efiiciently remove even the last traces of the contaminating ingredients from the acetic acid or other acid having a slowly moving ion. The cell chamber 8,

which may be constructed of material similar to that used in the above mentioned electrolytic cell, is fed through a conduit 9 by the impure acetic acid which flows into the cell in the direction of the arrow shown. The cathode surface 10 which lies close to the walls of the cell is separated from the anode surface 11 by the porous walls or cell 12. The

" aceticacid as it is slowly introduced into this cell gradually passes in the direction of the may be conducted in an elecemme arrows and du cell the anions, a rimarily e more rapidr' it'spassa through-the t ly moving amons, pass through the porous wall 12 intothe anode compartment 15. It-

is evident that the acetic acid solution as it flows to'the exit conduit 18 becomes freed from the contaminating ingredients, the metallic constituents being. plated u cathode surfaces 10. The separa such as the chloride or other'mineral acid ions are removed from the anode com artment through-the conduit 14. If lured,

n the anions" for ultimate purification or in case t e cell used does not remove the last ortions of the impurities from the acetic aci the partially passed through a second cell similar to the purified acid'from the conduit 13 may be rst. It is likewise possible that if the rous medium '12 be too porous a consi erable amount of acetic acid will be found in the anode com artment 15. It may, therefore,be

found advisable to pass the solution obtained from the anode compartment into a third cellsimilar to the first in order to recover. therefrom some of the occluded acetic acid.:

These additional cells are not shown as their attachment to the conduits 13and 14 and construct-ion is obvious from the above description.

The porous tube or walls with which we surround the anode surface is composed of some material which is-not attacked by the solution and while it resists theready passage of the acetic acid solution into the anode portion 'it, nevertheless, due to its porosit permits the transference of the very mobi e anions therethrough. Due to the slug ish mobility of the acetic acid ion itis en ent that it will be left behind in the cathode portion of the cell while the anode rtion will contain substantially all of'the c loride ions with, of course, a small percentage of the acetate ions. We have found that an anode cell of course alundum or similarly porous porcelain or other-pervious material may be.

em loyed.

. ur process for the purification of particularly acetic acid solutions will not be' restricted to that acid solely but may be employed with any' solute containing an 'anion Ill aving a relatively low mobility or any or ganic or inorganic acid solutions in which the anion has a very low mobility, such as that found in solutions containing aliphatic organic acids, or the mineral acids, such as boric or arsenious acid, in a solution of'which these ions may be present with a more rapidly moving anion. This process may likewise be utilized for the separation from solution of a mixture of solutes having cations of widely separating degrees of mobility. It is merely necessary in this instance to enca'se the cathode with the porous cell which is surrounded by the anode. An'example in which such a separation mama-1 is useful is for the separation of strontium This process is likewise useful for the sep aration of a mixture of salts in solution containing both a slowly and rapidly moving cation and anion. By superposing a porous medium before both the cathode and anode and electrolyzing, the slowly movin ions will remain in the central portion of t ecell and the rapidly moving cation and anions will be found transferred to their respective electrodes. An acid solution containing both zinc sulfate and ammonium cyanide can be readily separated by this process. The ammonium cyanide remaining in the central portion of the cell.

As the distance between the anode and the cathode surfaces is relatively small and althou h the resistance of the acetic acid solution 1s relatively high, the current density required to separate from the acetic acid solution the metallic and non-metallic impurities is found to be in theneighborhood of from .2 to .3 amperes per square centimeter for 30% acetic acid solutions containin mineral acids,for other solutions the optimum current density will vary with the particular solution.

As an example of the efiiciency of thi method of purification, and by which we will not be restricted either as to proportions or equivalents unless they be indicated in the appended claims, we havepurified an acetic acid solution of 30% acetic acid in approximately water which was recovered from a completed acetylating process and which contained both chlorides and copper salts by electrolyzing this acetic acid solution in acontainer as shown in Fig. 1, by applying thereto a current of 2 amperes and a voltage of from 20 to 84, giving a current density of .2 to .3 amperes per square centimeter. The chlorides and copper were completely removed as indicated by tests of the solution in the anode compartment and the cathode compartment; in 12 hours all the chlorides and the blue copper ion color was entirely discharged from the cathode compartment.

Various changes may be made in the type of electrolytic cell used for carrying out this purification reaction or in the type of solution purified providing that solution contains both a very mobile ion and an ion having a slower mobility with or without the presence of metallic salts therein without departing from this invention or sacrificing any of the advantages that may be derived from its use.

What we claim as our invention and desire to be secured by Letters Patent of the United States is:

1. A process for the purification of a solution comprising only electrolytes containing ions having a high mobility and ions 'having low mobility these ions having the same p0 a rity which comprises electrolyzing such solutlon by removing therefrom the more .mobile ions, these ions traversing but a relatively short path through a porous cell to 'their respective electrode.

2. A process for the purification of a solution 'comprising only electrolytes containmg anions having a high mobility and anions havlng low mobility which comprises electrolyzing such solution by removing therefrom the more mobile anions, these anions traversing but a relatively short path through a porous cell to the anode.

3. A process for the purification of solutlons comprising only electrolytes containing lOIlS whose difference in mobility is ap roximately 20% which comprises electro yzing the solution b removing therefrom the more moblle lons, t ese ions traversing but a relatively short path through a porous cell to their respective electrode;

4. A process for the purification of solutions comprlsing only electrolytes containing amons whose difierence in mobility is approximately 20% which comprises electrolyzlng the solution by removing therefrom the, more mobile anions, these anions traverslng but a relatively short path through a porous cell to the anode.

:5. A process for the purification of acetic ac d WhlCll comprises electrolyzing the acetic acld solution containing highly mobile ions by removing therefrom the highly mobile 10ns, these ions traversing but a relatively short path through a porous cell to the anode.

f3. process for the purification of acetic acid containing a mineral acid which comprises electrolyzing the acetic acid solution by removing therefrom the anion of the mineral acld, this anion traversing but a relatlvely short path through a porouscell to the anode.

:7. A process for the purification of acetic ac d containing hydrochloric acid which'comprises electrolyzing the acetic acid solution by removing therefrom the chloride ion, this chloride ion traversing but a relatively short path through a porous cell to the anode.

8., A process for the purification of acetic acid containing metallic and non-metallic impurities which comprises electrolyzing the acetic acid solution by removing therefrom the more mobile non-metallic anions, these anions traversing but a. relatively short; ath through a porous cell to the anode an" .the metallic impurities being plated on the oathode.

9. A process for the purification of acetic acid containing a mineral acid and metallic acetic acid solution by removing therefrom III the anions of the mineral acid, these anions traversing but a relatively short path through a porous cell to the anode, the metallic cations being plated on the cathode.

10. A rocess for the purification of acetic acid which comprises continuously feeding the acetic acid solution into an electrolyzing cell, removing from the cathode portion of the cell the more mobile anions to the anode com artment, the anions traversing but a relative y short path through a porous cell to the anode compartment, and continuously removing the purified acetic acid from the cathode portion and the separated anions from the anode ortion.

11. rocess for the purification of acetic acid v which comprises electrolyzin a 30% solution thereof containing hydrochloric acid and metallic impurities by subjecting it to a current density of approximately .25 amperes r square centimeter, the chloride ions pass- 1ng but a relatively short path through a porous cell to the anode and the metallic cations plating out on the cathode.

12. A process for the purification of a solution comprising only electrolytes containin anions having high and low mobility an cations having high and low mobility which comprises removing therefrom the more mobile ions, these ions traversing but a relatively short path through a porous cell to their respective electrodes.

Signed at Springfield, Mass, this 7th day of January, 1930, v

CHARLES S. WEBBER. S1gned at Rochester, New York, this 15th day of January, 1930.

CYRIL J. STAUD. HARRY LE B. GRAY. 

